Breakage and re-growth of flocs: Effect of additional doses of coagulant species

Abstract

Several polyaluminum chloride (PACl) coagulants were prepared, with different OH/Al ratios (B values), and characterized by Ferron assay. These were used in studies of floc formation, breakage and re-growth with kaolin suspensions under controlled shear conditions, using a continuous optical monitoring method. Particular attention was paid to the effect of small additional coagulant dosages, added during the floc breakage period, on the re-growth of broken flocs. The results showed that the re-growth ability was greatly dependent on the nature of the PACl species added as second coagulant. The re-growth ability of broken flocs was greatest when the second coagulant was PACl 0 (i.e. AlCl 3, with B = 0) and least with PACl 25 ( B = 2.5). In the latter case there was no effect on floc re-growth, irrespective of the initial coagulant used. PACls with intermediate B values gave some improvement in floc re-growth, but less than that with PACl 0. Additional dosage of PACl 0 gave re-grown flocs about the same size or even larger than those before breakage. The re-growth of broken flocs is significantly correlated with the species Al a (monomeric) and Al b (polymeric), as determined by Ferron assay. The amorphous hydroxide precipitate formed from PACl 0, (mainly Al a) can greatly improve the adhesion between broken flocs and give complete re-growth. However, for PACl 25, mostly composed of Al b, the nature of the precipitate is different and there is no effect on floc re-growth.